With both Medal of Honor games flopping, I can imagine they'll either return back to Bad Company or release Battlefield in a biennial form.
I really can't see them pumping out Battlefield games once a year. It's not the same kind of experience as a Call of Duty. They're getting longevity out of Battlefield 3 by releasing sizeable content packs over a prolonged period and that's more than enough to tie players to the game for 2 years. Releasing a Battlefield game every year would be a disaster.